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Note 14 - Asset Retirement Obligations - Reconciliation of the Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Balance at beginning of year $ 23,348 $ 20,856  
Revision in estimated cash flows 1,888 303  
Liabilities incurred 540  
Accretion expense 1,874 1,779 $ 1,198
Balance at end of year $ 27,110 $ 23,348 $ 20,856